Quality of Service (QoS) refers to resource reservation control mechanisms rather than the achieved service quality. Quality of Service is the ability to provide different priority to different applications, users, or data flows, or to guarantee a certain level of performance to a data flow. Quality of Service guarantees are important if the network capacity is insufficient, especially for real-time streaming multimedia applications such as Voice over Internet Protocol, online games and IP-TV, since these often require fixed bit rate and are delay sensitive, and in networks where the capacity is a limited resource, for example in cellular data communication.

An alternative to complex QoS control mechanisms is to provide high quality communication by generously over-provisioning a network so that capacity is based on peak traffic load estimates. This approach is simple and economical for networks with predictable and light traffic loads. The performance is reasonable for many applications. This might include demanding applications that can compensate for variations in bandwidth and delay with large receive buffers, which is often possible for example in video streaming. In networks typical of enterprises, however, the costs of increasing bandwidth can be substantial and over-provisioning is hard to justify. In these cases, QoS allows for prioritization of time-sensitive data without the expense of upgrading cabling and equipment.